Conversion of an internal combustion engine originally designed for a road vehicle into a marine-adapted version (including salt-water and fresh-water variants) to be run inside an isolated compartment is often carried out; but often not with great success.
A number of factors are relevant to inboard boat installation as compared with motor vehicles.
1. The enclosed housing in a boat—a structure often including combustible materials unlike the engine compartment of a motor vehicle—requires the engine to be kept cool. It is desirable that no part of the engine becomes hot (over about 125 deg C.) because an excessive temperature and amount of radiant or convective heat arising from an exhaust system is likely to char or cause a fire in any inflammable materials nearby. Marine regulations have been written to specify the limits of heating that are allowed.
2. When a boat engine is being used, it is typically operated at perhaps 70-85% of maximum capacity for long periods which is not often the case in a motor vehicle. More heat is evolved as a result than might be expected from vehicle experience.
3. On the other hand, an ample supply of fresh cooling medium (herein called raw water) is available as long as reliable circulation is assured. The primary dump of heat is made into raw water rather than air.
Ways to decrease the amount of heat given off by an exhaust manifold are known in the automotive industry. For example, a white coating or an insulating layer of ceramic mixture is applied to the exterior of the manifold. More simply, exhaust wrap may be wrapped completely around the manifold. Local overheating under the coating or wrap can lead to premature degradation of the manifold.
Most marine conversions include a water-cooled engine having a heat exchanger for dissipating heat from the engine itself into the ample supply of external cooling water. It is desirable that they include a water-cooled exhaust system. This specification does not describe placement of a catalytic converter (if any) but the skilled addressee will know how to install and maintain a catalytic converter within an appropriate part of the exhaust system.
A factor of relevance for the selected type of engine is that the input site for engine coolant (102A in FIG. 1) is situated at the rear of the engine while marine conversions reverse the general flow of exhaust gases to direct those gases toward the rear.